The Infamous Stringdusters have made a career out of bending the rules of bluegrass and stretching the boundaries of acoustic music, and with their sixth studio album, they not only mess around with their own formula but strike a blow for gender equality, at least within their own ranks. The title Ladies & Gentlemen refers to the album's concept -- the all-male Stringdusters invited a different female guest vocalist to appear on each of these 11 tracks, with each singing an original song that was written by the group with them in mind. (The sole exception is the closing number, "Hazosphere," an instrumental that features guest soloist Jennifer Hartswick on trumpet.) The group's songwriters reveal an inspired sense of casting here, wisely matching the material with the singers -- there's a slinky, jazzy groove to "Have a Little Faith" that suits Joss Stone well, Joan Osborne's soulful, organic approach meshes nicely with the dynamics of "Listen," Celia Woodsmith's smoky, evocative instrument was made for the rough-and-tumble mood of "Old Whiskey Bottle," "Ladders in the Sky" is a splendid vehicle for the unaffected clarity of Claire Lynch's vocal style, and Mary Chapin Carpenter is the wary but vibrant voice of experience on "Coming Back to You." And while the Stringdusters handily demonstrate their bluegrass bona fides on these recordings, there's more than a little blues, soul, jazz, and even a dash of pop in the songs featured here, and the picking is both technically proficient and musically thoughtful at every turn. The Infamous Stringdusters actually put themselves in the background on much of Ladies & Gentlemen, letting their guests take the center stage while they provide the support, but if the Stringdusters opted to be accompanists rather than the stars of the show on these sessions, their songs and effortless virtuosity make it clear they're every bit as talented as their friends. And given how many great singers they brought to this party, that's saying a lot.